Hard disk devices are designed to read data from/to write data to a rapidly rotating magnetic disk with a head flying thereover. In a manufacturing process of the disk, a projection called a media bump (hereinafter simply referred to as a bump) is sometimes formed on the surface of the magnetic disk erroneously. The size of the bump is a few hundreds of nanometers to a few micrometers in width and a few tens of nanometers in height. If the height of the bump is greater than the flying height of the head, the head contacts the bump, leading to deterioration or destruction of the head. Furthermore, data cannot be recorded on the part of the disk where the bump is. To avoid these situations, magnetic disks are inspected before shipping in order to detect bumps on the disk and record the positions of the bumps, and the flying height is controlled to prevent the head from contacting the bumps on a user's device.
However, the flying height of the head used in the bump detection inspection may be fixed, and if there are relatively large bumps on the magnetic disk, the head contacts the bumps many times and deterioration or destruction of the head is inevitable. In consideration of this point, the flying height of the head may be set greater (that is, the head may be set higher) than that in an actual read/write operation. However, in this case, gap between the magnetic disk and the head increases, and accordingly, minute or low bumps cannot be detected and the accuracy of the detection cannot be sufficient.